1. Field of the Invention
The present invention relates to a speed regulator for a brushless DC motor according to the precharacterising clause of claim 1.
2. Discussion of Related Art
Such a regulator is known in the state of the art for regulating the voltage supplied to a DC motor. In the DC motor there are a stator, a rotor and an electromagnet, the electromagnet being in the form of a winding around the rotor component or inside the rotor component. The rotor consists of a number of permanent magnet poles located uniform distances apart on a circle, the stator poles generating a permanent magnetic field. The stator is either inside or outside this circle. During use an alternating magnetic field is generated in the electromagnet by the voltage supplied, the rotor executing a rotation under the influence of the alternating magnetic field of electromagnet and stator.
In the state of the art activation of the electromagnet by means of rectangular pulses is employed, the voltage being supplied to the electromagnet in the form of a rectangular pulse. Because of the transient that arises in the drive torque of the motor when a rotor pole passes by a stator pole, a variable-speed brushless DC motor exhibits vibration while the motor is turning. This vibration leads to the production of noise by the DC motor. However, such noise production is undesirable for many applications where speeds of revolution of less than approximately 200 rpm are usually used.
It is known to provide a voltage regulator in the DC motor such that the transient of the drive torque is minimised at a (pre-)determined speed of revolution. For this purpose the speed regulator is provided with a Hall sensor that is positioned some distance away along the path of the rotor next to one of the stator poles. The regulator is so set that, for said set distance and for a predetermined speed of revolution, the signal that the Hall sensor generates as a result of the rotor field through the Hall sensor ensures that the reversal of polarity of the magnetic field proceeds in such a way that the transient of the torque of the motor while the rotor is passing by the stator is minimal.
A disadvantage of this regulator according to the state of the art is that vibration is substantially suppressed only at mainly one speed of revolution. At other speeds of revolution this regulator does not function well; the torque transient is not then minimal.
As an alternative to achieve a sufficiently usable suppression of vibration at several speeds of revolution, a voltage regulator is known that employs rectangular pulses of variable height, the height of the rectangular pulse depending on the desired speed of revolution of the motor. In order to reduce the vibration at a set speed of revolution, the pulse height of the rectangular pulse is lowered, so that the field strength is lower and the transient smaller. Vibration does indeed decrease by this means, but does not reach the values that are theoretically possible.
One aim of the present invention is to provide a speed regulator for a brushless DC motor that is capable of substantially reducing the transient in the motor torque at any arbitrary speed of revolution.